Amplifier circuit



Feb. 17, 1942. J HAANTJES 2,273,519

AMPLIFIER C IRCUIT Filed June 25, 1940 INVENTOR. JOF/AN HAANTJES ATTORNEY Patented Feb. 17, 1942 AMPLIFIER CIRCUIT Johan Haantjes, Eindhov'en, Netherlands, assignor to Radio Corporatio not America, a corporation of Delaware Application June 25;"1940, Serial No. 342,195

In the Netherlands September a, 1939 Claims.

The invention relates to a circuit arrangement'which comprisesan impedance constituted by the input-impedance of a low-pass or-bandpass filter of the 1ror T-type respectively.

It is already known to use such impedances in amplifiers such as, for instance, television amplifiers by which a large range of frequencies is to be uniformly amplified. In these well-known circuits the impedance connected in the anode circuit of an amplifying tube is constituted by the input-impedance of a low-pass filter section of the fundamental 1r-type which consequently consists of an inductance L1 in the series-branch and capacities 02 in each of the two shuntbranches. The output terminals are terminated by a resistance which is matched to the imageimpedance between the output terminals. In these circumstances the input-impedance corresponds to the image-impedance and this be-' haves in the transmission band as a resistance whose value increases as the frequency is higher, which consequently would yield an amplification dependent on the frequency. In order to render the absolute value of the input-impedance and consequently also the amplification independent with respect to frequency it has furthermore been proposed already to connect, in parallel with the input terminals, an additional condenser 02 corresponding to the shunt-condenser of the filter network between these terminals. As will be shown hereinafter the absolute value of they input-impedance of the filter corresponds in this case to the'nominal value R of the image-impedance so that the amplification determined by the input-impedance is constant for all frequencies in the transmission band. It appears that the product of the band-width (i. e. the width of the range of frequencies transmitted) and amplification which can be achieved by means of such an amplifier is inversely proportional to the value of the shunt condenser /202 of the filter network and'since the minimum value of thiscondenser is limited by the anodecathode capacity of the amplifying tube connected in parallel with the input-terminals of the filter network the attainable amplification can be raised only at the cost of the band-width and conversely. I

The present invention has for its object to enhance the input impedance of low or band-pass filters so that when, for instance, it is used as an impedance in the anodecircuit of the amplifying tube the product of band-width and amplification is larger than that which could hitherto be attained.

vention; Fig. 2 shows a In a circuit arrangement-including an impedance constituted by the input-impedance of a low-pass or band-pass filter of the 1ror T-type, respectively, whose image-impedance between the input terminals, the nominal amounts to R, corresponds to the image-impedance of the basic type of low or band-pass filters and whose output circuit is terminated by an impedance matched to the image-impedance between the output terminals, vention this purpose is attained by connecting in parallel with the shunt-impedance 2Z2 of the 1r-type lying between the input terminals an impedance and a negative resistance -R1, respectively, and' by connecting in series with the series-impedance Z1 of the T type an impedance nZ'1 and a negative resistance -R1, n being given by the expression R' n .\/1 R1- The invention will be more clearly understood by reference to the accompanying drawing,

given by way of example, wherein Fig. 1 shows a symmetrical low-pass or band-pass filter network of the 1r-type, embodying the present included a low-pass filter network of the fundamental 1r-type; Fig.- 3 is a circui similar to that of Fig. 2, but with the present therein; Fig. 4 is tion disclosed in Fig. 3; Fig. 5 shows the application of the invention to a band-pass filter network of the w-type; and Fig. 6 shows a symmetrical low-pass or band-pass filter network of the T-type in accordance with the present invention.

Fig. 1 shows a symmetrical low or band-pass filter networkconsisting of a series-impedancev Z1 and two shunt-impedances 2Z2. It is supposed that the construction'of the impedances Z1 and 2Z z ,corresponds'to that of the fundamental type of the low or band-pass filters or are inferred therefrom by the well-known m1r-transformation. Starting therewith the image-im-- pedance Z1r', between the input-terminals I, 2

and, on account of symmetry of the filternetwork, the corresponding image-impedance bevalue of which according to the inknown amplifying circuit in the output circuit ofwhich there is ininvention embodied a modified form of the inventween the output-terminals 3, 4 is given by the expression:

In this expression R represents the nominal value of the image-impedance, a: in the case of a low-pass filter being given by sponds to the nominal value of the image-impedance Zr.

According to the invention anv impedance 22 and a negative resistance R1 are connected between the input terminals I, 2 of the filter network. The impedance Z1 measured between the input-terminals then amounts to Now it is known from filter-theory that for low and band-pass filters of the fundamental type or filters inferred therefrom according to the m1r transformation After substitution of the Expressions l and 3 in the Expression 2:

= RRI R141 27 R I The absolute value of the input-impedance Zl consequently amounts to RR, /1? zRR 1 :z; +R(1x +n x Ri The expression /1--::: forming part of the denominator may be developed in the following series I According to the invention 12. is chosen so as to fulfil the condition:

hence This expression includes the frequency-dependent magnitude :2, i. e. that theabsolute value of the input-impedance Z; is not entirely constant in the transmission band. In practice, however, the deviation makes itself felt only at the edge of the range of transmission.

If, for instance,

whence it results for a low-pass filter that at the limiting frequency Z1 has fallen to 91.7% of the value at x=0 and for x=0.8 to 96.3%. Thus, in the Expression 4 above,

i 4 1 may, to a first approximation, be supposed to be equal to and very much smaller than 1, hence R (Zi) Since n is smaller than 1 the absolute value of the input-impedance is higher than the inputimpedance of the well-known filter referred to above in which the absolute value of the inputimpedance amounts to R. Consequently, -when the impedance Z1 is used as an anode-impedance of an amplifying tube this circuit permits a larger product of band-width and amplification to be attained than in the case of the known circuit, which will be more fully explained by reference to the circuit arrangements shown in Figures 2 and 3.

Fig.2 shows the well-known amplifying circuit including an amplifying tube 5 of the pentodetype, whose anode-impedance is constituted by a low-pass filter network of the fundamental 1rtype consisting of an inductance L1 in the series branch and a condenser impedance z a 41-15 a condenser being connected in parallel with the input-terminals I, 2. From filter theory it follows that c =f =limiting frequency) so that whence it follows that Z|=R.

If the mutual conductance of the amplifying tube is S the amplification amounts to V=S.R and sinceithe band-width is given by l of; it follows that the product of band-width and amplification is In this case the total capacity between the input terminals of the filter amounts to C2 and in order to render the product Vii as large as possible this capacity should be made as small as possible, which is the case when this capacity is constituted by the anode-cathode capacity Cak of the amplifying tube 5. Inthis case:

'7 G. We shall now consider the amplifying circuit shown in Fig. 3, wherein the anode-impedance of the amplifying tube 5 is an impedance according to the invention. This impedance is constituted by the input-impedance of a low-pass'filter similar to that used in the circuit shown in Fig. 2, but now a condenser v and a negative resistance R1 is connected in parallel with the input terminals I, 2,

holding good, as has been stated above, for the absolute value of the input impedance R and consequently for the amplification The band-width amounts to 1 fl ,x 021R so that we eventually have v S 1 I -fl m (6) for the product of bandwidth and amplification. The total input capacity of the filter amounts to and this is chosen so as to correspond again the anode-cathode capacity Cat of the amplifying tube, hence 2 I C C',,,, The latter expression substituted in (6) yields s n+1 1 Upon comparison of the Expressions 5 and 7 it is found that when making use of the circuit shown the improvement The negative resistance -R1 may, for instance,

uct of band-width and amplification with respect to the known circuit amounting to a factor Zn In the case or amounts to a factor 6.

be constituted by the internal resistance between anode and cathode of a. screen-grid tube, whose anode voltage and screen-grid voltage are adjusted so that the internal-resistance is. negative. However, this circuit has the drawback that the capacity lying between the input terminals of the filter is increased by the capacity between anode and cathode of the screen-grid tube, which, as appears from what has been said above, results in that the product of bandwidth and amplification to be attained becomes smaller.

However, the so-called secondary-emission tube is a suitable expedient to secure the negative resistance -R1 without experiencing the drawback just referred to. This is because the internal capacity between cathode and the sec,-

ondary electron emitting electrode to which the input terminals of the filter are connected is muchless than the capacity between cathode and anode of a screen grid tube operating as a negative resistance device. stance the input terminals of the filter are connected between cathode and anode.

Fig. 4 represents a circuit arrangement comprising a secondary-emission tube. This tube 6 includes a cathode I,v a control-grid 8 to which are supplied the oscillation to be amplified, an auxiliary cathode ,9 emitting secondary electrons upon bombardment by electrons and an anode l0. Between the auxiliary cathode 9 and the l in Fig. 3 we obtain an improvement of the prodcathode I is connected an impedance constituted by the input impedance of a low-pass filter whose construction corresponds to that of the circuit shown in Fig. 3; According to the invention a condenser is connected in parallel with the input terminals I, 2. By a suitable choice of the direct voltage Eh the-internal resistance between the auxiliary cathode and the cathode may be ad- R Z r I /1-12;

depends on the frequency.

,The method used in practice to terminate the output circuitby a constant resistance R cor- In the latter in-- responding to the nominal value of the imageimpedance gives rise to deviations from the calculated result which in amplifying circuits, for

instance, is manifest by falling off of the amp1i-- fication to a higher degree at the boundaries of the transmission band. However, this drawback can be met in a known manner by arranging for the filter to end with a half section of the mtype which, as is well known, exhibits a more constant characteristic of the image-impedance in the transmission band.

As has already been observed the explanation of the invention given by reference to Fig. 1 also applies for band-pass filters. In the special case of a band-pass filter of the fundamental type the impedance Z1 in the series-branch consists of the series-connection of an inductance L1 and a capacity C1, as more clearly appears y from Fig. 5, the shunt-impedances 222 being constituted by the parallel-connection of an inductance 2L2 and a condenser Q 2 According to the invention an impedance and a resistance -R1 are connected in parallel with the input terminals 1, 2 where the input impedance Z1 of this band-filter ap-' proximately corresponds again to n Consequ'entlyband-pass filters permit the obtainment of the same advantages as those stated by reference to Fig. 3 for a low-pass filter.

In this circuit arrangement, also, it is advisable to connect the resistance R over a half section of the m-type to the output terminals 3, 4.

When using a low-pass filter or a band-pass; filter it is not necessary that this should consist of a complete section. The considerations stated above also apply when using a half-section. Furthermore, it is once more observed, that, al-

though in Figures 3 and 5 use is made of a lowpass filter and a band-pass filter of the fundamental type, respectively, the invention is not limited to this kind of filters and that low-pass filters or band-pass filters inferred from the basic type by means of nix-transformation may be used.

After explaining the invention while making use of filters of the 1r-type it is superfluous to give an explanation for the case in which the impedance'i's" constituted by the input impedance of a low-pass or band-pass T-filter of the basic type or of a .type inferred therefrom by means impedance between the input-terminals, the

nominal value of which amounts to R, corresponds to the image impedance of the basic type of filter and whose output circuit is terminated by an impedance matched to the image impedance between the output terminals, in which in parallel with the shunt impedance 2Z2 of the 1r-type lying between the input-terminals are connected an impedance ggz and a negative resistance R1, n being given by the expression:

R 7L -\/1 H1 2. An amplifier circuit according to the invention defined in claim 1, wherein the electron discharge tube includes an electrode which emits secondary electrons, and the input terminals of by an impedance matched to' the image impedance between the output terminals, and in which in series with the series-impedance /221 of the T-type are connected an impedance /2nZ1 and a negative resistance R1, n being 'given by the expression:

4. An amplifying circuit arrangement comprising an impedance constituted by the input impedance of a standard derived type of filter Whose image impedance between the input terminals has the nominal value R which is equal to the image impedance of the corresponding basic type of filter and whose output circuit is terminated by an impedance matched to the image impedance between the output terminals, said circuit arrangement having a negative resistance element connected to the initial reactance element of said filter to produce therewith a composite impedance network having a negative power factor, that is, a negative ratio of real component to magnitude of imaginary component, determined by the ratio of the impedances an ,-increased product of said elements, and wherein the magnitude of the impedance of said initial element is so altered as to reduce the magnitude of said power factor by the factorn+ 1 the value of n being given by the expression nected to the initial reactance element of said filter whereby to produce therewith a composite element havinga negative power factor, and

further reactance connected to said initial reactanee element and so chosen as to alter solely the magnitude of said initial reactance at every R being frequency, said alteration of magnitude being of such amount as to cause the power factor of the so altered composite element to be reduced to 72+ 1 times the first mentioned power factor, where the magnitude of said terminating resistance and R1 being themagnitude of said negative resistance.

JOHAN HAA-NTJES. 

